In recent years, a high-performance lithium secondary battery or the like has been increasingly demanded for a portable information terminal, a portable electronic instrument, a domestic small power storage device, a motorcycle, an electric vehicle, and a hybrid electric vehicle that utilize a motor as the power source, and the like. The term “secondary battery” refers to a battery that can be charged and discharged. A secondary battery has been used in a wide variety of applications, and a further improvement in safety and performance has been desired.
An electrolyte that exhibits high lithium ion conductivity at room temperature has been generally limited to an organic electrolyte. An organic electrolyte is flammable due to inclusion of an organic solvent. Therefore, leakage or ignition may occur when using an ion-conductive material containing an organic solvent as an electrolyte for a battery. Moreover, since the organic electrolyte is liquid, conduction of counter anions occurs in addition to conduction of lithium ions. Therefore, the lithium ion transport number is equal to or less than “1”.
An inorganic solid electrolyte is nonflammable, and is highly safe as compared with the organic electrolyte which is usually used. However, since the electrochemical performance of the inorganic solid electrolyte is inferior to some extent as compared with the organic electrolyte, it is necessary to further improve the performance of the inorganic solid electrolyte.
In view of the above situation, a sulfide solid electrolyte has been extensively studied.
For example, sulfide glass having an ion conductivity of 10−3 S/cm (e.g., LiI—Li2S—P2S5, LiI—Li2S—B2S3, and LiI—Li2S—SiS2) was discovered in the 1980's as a lithium-ion-conductive solid electrolyte having high ionic conductivity.
These electrolytes are normally solids, and ground (homogenized) by milling to obtain solid electrolyte particles (Patent Document 1). An electrolyte may also be obtained by a heat melting method at a high temperature (Patent Document 2).